It's been a while since my last post, so it's about time for an update.
Since I stopped with the Opiliones project I've still been working on my 3D printering. I'll call it an Additive Manufacturing machine (AM) because I think the name 'printer' indicates it's as simple as pushing a button and having a physical product within the minute. And that's simply not true, It's manufacturing and not printing.
I've been working with LinuxCNC, a BeagleBone Black and a BeBoPr-Bridge board (Cape) since end of december last year. LinuxCNC (and it's recent fork Machinekit) is software for controlling machines...
Recently there has been a lot of development in this area, IMO one very important one is the blending of many short lines (very common output of slicing software) into smooth motion. This is important because when you are making a product, the quality of the extrusion benefits from a smooth constant motion.
I've been hacking away recently in making the configuration files fit my machine. One thing that I use when setting up a machine is adjusting the flow rate during running a program. But this is difficult if the extrusion is controlled by position. You'd have to remember the current position and from that point on you'd need to multiply the positions from that offset point. A lot of calculations and it's currently not available. During the discussions on the Machinekit group the idea came to make the extrusion dependant on the actual nozzle speed.
One of the fun things about being open (source) is that when you want to have extra functionality you can add it yourself. It takes some learning curve sometimes (LinuxCNC, linux, working with git, pull requests etc) but in the end you can actually improve and have exactly what you (think you) need.
So I ended up with:
- Changing the configuration files so that LinuxCNC calculates speed of the extruder based on the nozzle speed (the extruder axis is velocity controlled instead of position controlled).
- Adding functions for setting width of the line being extruded and the height of that line (the current layer).
- (Un)linking the extruder with the nozzle velocity.
- Writing scripts that post processes the slicer g-code output (remove all A-axis positions) and inserts the dimensions into the g-code.
Now you don't necessarily have to slice, you can draw on your bed, like with Logo Turtle on the MSX 1 (If you don't know what an MSX 1 is, you probably are a lot younger than me) put the pen down, draw a line, put the pen up (but in G-code).
What's so extremely powerful is the HAL module. You want something added? Then take the blocks you need, multiplying, limiting, etcetera and virtually rewire your machine behaviour.
Because the HAL is so powerful it took little effort to add the bonus function: Live nozzle pressure adjustment. An extra adjustment of the extruder for the current speed. Why? Because inside the nozzle there is a pressure depending on the extruding velocity. So when you start extruding (v=0) you have to build up pressure (having too little plastic while accelerating) and when decelerating you have too much pressure, resulting in a release when you are stopped. This is something you frequently see on sides of the product. See picture below of standard blobs, with plain and simple extrusion. The 3 lines at the bottom makes an "S" movement with disconnecting the extruder speed with the nozzle on the vertical movement, but without retracting. The top 3 lines is the "S" movement, but with retracting.
Because it just takes some virtual rewiring of the HAL i've added the derivate of speed function (ddt) and used that as an input for a lookup table (lincurve, thanks Andy) which adds velocity when accelerating, adds none during constant speed, and subtracts velocity while decelerating. Effectively taking care of the pressure hysteresis inside the nozzle. Want to have other/more specific/finegrained control then you just insert points in the lookup table. Imagine doing that to a elastic bowden extruder? I have my extruder mounted on the effector, with just 8 cm between the drive wheels and the nozzle of my E3D hot-end but even there this phenomenon is there. See result below after adjusting, no blobs at the end of the lines.
here's a comparison of the "normal" extruding at the left, and the "pressure adjusted velocity controlled extrusion" at the right.
And last but not least velocity controlled extrusion in action.
update 1: see this post how you can use this for PCB additive manufacturing
update 2: see this post how you can lay down wire in a pattern
update 3: I've updated the velocity extruding. Go here if you're interested
Thanks a lot to the guys at the Machinekit and LinuxCNC users list.
More info?
Until merged in the main branch, my working branche on the velocity extruding is here.
https://github.com/machinekit/machinekit
Have fun!
